Moon, showing bum in front of polite society (6)
I believe the answer is:
phobos
'moon' is the definition.
(I've seen this before)
'bum in front of polite society' is the wordplay.
'bum' becomes 'hobo' (synonyms).
'in' indicates putting letters inside.
'front of' suggests taking the first letters.
The first letters of 'polite society' is 'ps'.
'hobo' going inside 'ps' is 'PHOBOS'.
'showing' acts as a link.
(Other definitions for phobos that I've seen before include "another world" , "heavenly body" , "Satellite of Mars" , "Planetary moon" , "Moon of Mars" .)